1. Field of the Invention
The present invention relates to a method for the synthesis of non-zeolitic molecular sieves. More particularly, the method relates to non-zeolitic molecular sieves which may be synthesized from an aqueous reaction mixture having a Al.sub.2 O.sub.3 :P.sub.2 O.sub.5 molar ratio greater than 0.3.
2. Description of the Related Art
Recently, there have been reported several classes of microporous compositions which are not zeolitic, and which will collectively be referred to hereinafter as "non-zeolitic molecular sieves", which term will be more precisely defined hereinafter. These materials are formed from AlO.sub.2 ! and PO.sub.2 ! tetrahedra and have electrovalently neutral frameworks, for example, as in the case of silica polymorphs and include the crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440 issued Jan. 12, 1982 to Wilson et al., the disclosure of which is incorporated herein by reference.
Classes of aluminophosphate compositions comprise AlPO.sub.4, SAPO (silicoaluminophosphate), MeAPO (metalloaluminophosphate), and ElAPO (nonmetal substituted aluminophosphate families). Uses for crystalline aluminophosphate compositions have included molecular sieves, catalysts, and catalyst carriers.
U.S. Pat. No. 4,440,871, issued on Apr. 3, 1984 to Lok et al., the entire disclosure of which is incorporated herein by reference, describes a class of silicon-substituted aluminophosphate non-zeolitic molecular sieves which are both microporous and crystalline. These materials have a three-dimensional crystal framework of PO.sub.2 !, AlO.sub.2 ! and SiO.sub.2 ! tetrahedral units and, exclusive of any alkali metal or calcium which may optionally be present, an as-synthesized empirical chemical composition on an anhydrous basis of: EQU mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3, the maximum value in each case depending upon the molecular dimensions of the templating agent and the available void volume of the pore system of the particular silicoaluminophosphate species involved; and "x", "y", and "z" represent the mole fractions of silicon, aluminum and phosphorus, respectively, present as tetrahedral oxides. The minimum value for each of "x", "y", and "z" is 0.01 and preferably 0.02. The maximum value for "x" is 0.98; for "y" is 0.60; and for "z" is 0.52. These silicoaluminophosphates exhibit several physical and chemical properties which are characteristic of both aluminosilicate zeolites and aluminophosphates.
U.S. Pat. No. 4,943,424, issued on Jul. 24, 1990 to Miller the entire disclosure of which is incorporated herein by reference, describes a crystalline silicoaluminophosphate molecular sieve (designated therein as "SM-3") having an as synthesized composition empirical chemical composition on an anhydrous basis of: EQU mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from 0.02 to 2, and "x", "y", and "z" represent the mole fractions of silicon, aluminum and phosphorus, respectively, present as tetrahedral oxides. The minimum value for "x" is 0.02; for "y" is 0.24; and for "z" is 0.24. The maximum value for "x" is 0.51; for "y" is 0.50; and for "z" is 0.51.
U.S. Pat. No. 4,500,651, issued to Lok et al. on Feb. 19, 1985 the entire disclosure of which is incorporated herein by reference, describes a class of titanium-containing non-zeolitic molecular sieves whose chemical composition in the as-synthesized and anhydrous form is represented by the unit empirical formula: EQU mR:(Ti.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Ti.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of between zero and about 5.0; and "x", "y" and "z" represent the mole fractions of titanium, aluminum and phosphorus, respectively, present as tetrahedral oxides.
U.S. Pat. No. 4,567,029, issued on Jan. 28, 1986 to Wilson et al., the entire disclosure of which is incorporated herein by reference, describes a class of crystalline metal aluminophosphate non-zeolitic molecular sieves (designated therein as "MeAPO") having three-dimensional microporous framework structures of MO.sub.2,! AlO.sub.2 ! and PO.sub.2 ! tetrahedral units and having an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(M.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (M.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; "M" represents at least one metal of the group magnesium, manganese, zinc and cobalt; and "x", "y", and "z" represent the mole fractions of the metal "M", aluminum and phosphorus, respectively, present as tetrahedral oxides.
U.S. Pat. No. 4,544,143, the entire disclosure of which is incorporated herein by reference, describes a class of crystalline ferroaluminophosphate non-zeolitic molecular sieves having a three-dimensional microporous framework structure of FeO.sub.2,! AlO.sub.2 ! and PO.sub.2 ! tetrahedral units and having an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(Fe.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Fe.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; and "x", "y" and "z" represent the mole fractions of the iron, aluminum and phosphorus, respectively, present as tetrahedral oxides.
U.S. Pat. No. 4,686,093, issued on Aug. 11, 1987 to Flanigen et al., the entire disclosure of which is incorporated herein by reference, discloses a class of crystalline non-zeolitic molecular sieves designed therein as "FCAPO" (denoting "Framework Constituents"), having a three-dimensional tetrahedral framework composed of AlO.sub.2 !, PO.sub.2 !, and MO.sub.2 ! units and having an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(M.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (M.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; M represents at least two elements present in the tetrahedral units selected from the group consisting of arsenic, beryllium, boron, chromium, gallium, germanium, lithium, and vanadium, and "x", "y" and "z" represent the mole fraction of "M", aluminum, and phosphorus, respectively, present as tetrahedral oxides.
U.S. Pat. No. 4,973,785, issued on Nov. 27, 1990 to Lok et al., the entire disclosure being incorporated herein by reference, discloses a class of crystalline non-zeolitic molecular sieves, designated therein as "ELAPSO". The ELAPSO compositions are formed with elements capable of forming framework oxide units in the presence of AlO.sub.2 !, SiO.sub.2 ! and PO.sub.2 ! tetrahedral units where element "EL" is at least one element capable of forming a three-dimensional oxide framework in the presence of aluminum, phosphorus and silicon oxide units, and has a mean "T-O" distance in tetrahedral oxide structures of between about 1.51 Angstroms and about 2.06 Angstroms, has a cation electronegativity between about 125 kcal/g-atom and about 310 kcal/g-atom and is capable of forming stable EL-O-P, EL-O-AL, and EL-O-EL bonds in crystalline three-dimension al oxide structures.
Typically, the aforementioned classes of non-zeolitic molecular sieves are synthesized by hydrothermally crystallizing a hydrous gel made from a substantially homogeneous aqueous reaction mixture containing reactive sources of aluminum, phosphorus, silicon (in the case of the silicoaluminophosphates) and the other element(s), if any, required in the molecular sieve. The reaction mixture also preferably contains an organic templating, i.e., structure-directing, agent, preferably a compound of an element of Group VA of the Periodic Table, and/or optionally an alkali or other metal. The reaction mixture is generally placed in a sealed pressure vessel, preferably with an inactive metallic surface or alternatively, lined with an inert plastic material such as polytetrafluoroethylene and heated, preferably under autogenous pressure, at a temperature between 50.degree. C. and 250.degree. C., and preferably between 100.degree. C. and 200.degree. C., until crystals of the non-zeolitic molecular sieve product are obtained. Usually this is for a period of from several hours to several weeks. Effective crystallization times from about 2 hours to about 30 days are generally employed. The molecular sieve is recovered by any convenient method, for example, centrifugation or filtration.
In preparing the hydrous gel, it has been reported by Romarneni and Roy (The Role of Gel Chemistry in Synthesis of Aluminophosphate Molecular Sieves, Zeolites, vol. 11, February 1991, pp. 142-148) that the gel's chemistry is essential in the aluminophosphate's formation. On a laboratory scale where reagents are readily mixed, the crystallization methods described in the aforementioned patents have been effective in producing non-zeolitic molecular sieves in high yields. However, for larger scale preparations, synthesis techniques require the constituents be combined over a substantially longer period of time. When the aluminum source is added to the phosphorus source there is a voluminous coagulum of an aluminum specie or species; resulting in a final mixture that is thick, viscous, and not readily dispersible. Therefore, there is a need for a method for producing non-zeolitic molecular sieves in quantities sufficient for commercial applications, and that avoids the problematic aluminum coagulation.
As used herein the terms "coagulum" and "precipitate" are used interchangeably and refers to the separation and binding together of the reagent, in solid form, from the solution.
Among other factors, when a crystalline, non-zeolitic molecular sieve is synthesized from a reaction mixture having a Al.sub.2 O.sub.3 :P.sub.2 O.sub.5 molar ratio greater than 0.3, it has now unexpectedly been found that it is critical that at least some of the templating agent(s) be added to the aqueous reaction mixture before a significant amount of the aluminum source is added. This reduces the precipitation of the aluminum and thereby the formation of a viscous gel.
It is, therefore, the principal object of the process embodiment of the present invention to limit, and preferably avoid, the viscous precipitate that forms during preparation of crystalline, non-zeolitic molecular sieves having a three-dimensional microporous framework structure comprising AlO.sub.2 ! and PO.sub.2 ! units. This and other objects are accomplished by the invention set forth in the description and examples below.